1. Field of Invention
The present invention relates to chromatographic systems and techniques of operation thereof and, more particularly, to gas chromatographic systems, subsystems, and techniques of operation involving heating and/or cooling chromatographic columns.
2. Background of the Invention
In existing chromatographic analysis, separation of components of an analyte typically occurs as the analyte moves through a column. The time for the components to traverse the column typically depends on the respective affinities of the components relative to the stationary phase. Because affinity is generally dependent on temperature, the temperature of the column is preferably controlled in a particular manner. Existing columns may further have temperature control which provides temperature regulation and/or ramping up of column temperature from a first temperature to a second higher temperature. Additionally, temperature control of a column may further include the need to ramp column temperature from a second higher temperature to a first lower temperature. For example, existing chromatography protocols typically start at low temperatures to provide sufficient resolution among the lighter components to be analyzed and then ramp up the temperature to elute the heavier components under analysis. For example, in existing chromatographic systems the initial column temperature may be slightly above ambient room temperature, for example, at about 40° C. During chromatographic separation, the temperature of the column may be increased at a predetermined rate and/or according to a predetermined schedule, resulting in a second higher column temperature. At the end of the analysis the column is preferably cooled as cooling the chromatographic column decreases the time between analyses, especially when the chromatographic systems are operated with one or more temperature increases.
The temperature of a column, and time spent at a predetermined temperature, is further related to the analyte under analysis. For example, chromatographic separation and analysis of a gas sample can generally be achieved in a shorter time and with lower maximum temperature to elute all components as compared to separation and analysis of heavy components of oils. Following the completion of the analysis, regardless of the analyte under analysis, a return of the column to a lower temperature is generally required. In light of the requirement, higher maximum temperatures which occur through temperature ramping for heavy components of an oil sample generally translate into longer cooling times before starting a subsequent analysis.
As understood by one skilled in the art, existing cooling methods include the use of forced convection utilizing ambient air. In some cases, however, forced convective cooling by ambient air is not possible, e.g., when the ambient air temperature is above the column temperature. Additionally techniques such as radiative or convective cooling are also employed in the prior art, but such systems are generally not practical in various environments. In a downhole environment, these existing techniques may not be appropriate or may be impossible when compared to conductive cooling techniques.
Further, attaching a cooling system directly to the column can create undesirable consequences because the attached cooling system increases the effective thermal mass of the column. When the columns are heated with a temperature ramp, the cooling system will also be heated. When the cooling system is needed, it will take longer to achieve the initial temperature due to the increased mass of the column. A larger thermal mass consequently requires a greater heating burden to achieve a particular rate of temperature increase. The increased power requirement may be a significant, especially in power limited environments such as a downhole environment. Further, the maximum allowable temperature of some cooling system components may below an operating temperature of one or more columns. For example, the maximum allowable temperature of Peltier coolers is typically about 225° C., thus the operating temperature of the column would be limited to below this temperature.